The invention relates to electric discharges, and more particularly to controlling the temperature of the medium in which the discharges take place.
The incandescent lamp is an often-used source of lighting in many homes and businesses. However, its light emitting element evaporates and becomes weak with use, and hence is easily fractured or dislodged from its supports. Thus, the lifetime of an incandescent lamp is short and unpredictable. More importantly, the efficiency of an incandescent lamp in converting electrical power to light is very low.
Discharge lamps, in which light is generated by an electric discharge in a gaseous medium, are generally more efficient and durable than incandescent lamps. See U.S. Pat. No. 4,010,400 issued Mar. 1, 1977 to Hollister.
As is known in the art, the efficiency of the discharge lamp depends on the temperature of the coldest spot ("the cold spot") of the gaseous medium. The discharge lamp efficiency reaches its maximum at a certain cold spot temperature Tm, between 30.degree. C. and 40.degree. C. for some lamps. See, for example, Netten and Verhiej, QL Induction Lighting (Philips Lighting B. V., 1991, printed in the Netherlands). Thus to maximize the efficiency, it is desirable to keep the cold spot temperature at the value Tm. However, the heat from the lamp can raise the cold spot temperature well above Tm. For example, in lamps with Tm below 40.degree. C., the heat can raise the cold spot temperature above 100.degree. C. Thus there is a need for a discharge lamp in which the cold spot temperature can be controlled so as to be closer to the value Tm.
Further, it is desirable to be able to easily measure the cold spot temperature in order to determine what factors bring the cold spot temperature closer to value Tm.